Sony. Sony Semiconductor has a long history of sensor development, and the sensors in Sony cameras are simply the latest iteration of a long chain of APS-sized and full frame sensors they've made.
A corollary question tends to be: are these the same sensors as Nikon uses in some of their cameras? That’s a tricky question. Sometimes the answer is yes, sometimes Nikon has tweaked the sensor in some way (often in the microlens/Bayer filtration layer). Moreover, many Sony sensors have multiple ways that they can be “driven,” and Nikon and Sony tend to differ in how they do this.